1. Field of the Invention
The present invention relates generally to electronic oscillator circuits and more particularly relates to a voltage controlled oscillator circuit having both low phase noise and wide tuning bandwidth.
2. Description of the Prior Art
Oscillator circuits, including voltage controlled oscillator circuits, are well known in the prior art. Typically, an oscillator circuit includes at least one active device and a resonator circuit coupled to the active device. The resonator is coupled to the active device such that the active device is capable of oscillating only at a specific frequency determined by the resonator. This results in a signal being generated at the frequency which satisfies the oscillation conditions.
In a voltage controlled oscillator (VCO), the frequency of oscillation is controlled by electronically altering the point of resonance of the resonator circuit. Typically, a variable resonator circuit includes at least one variable capacitance diode (varactor). The frequency of operation is set by applying a bias voltage to the varactor to alter the capacitance of the varactor diode. This change in capacitance alters the net reactance of the resonator and alters the frequency of operation of the oscillator circuit.
FIG. 1 illustrates a first resonator circuit which is known in the prior art. This conventional resonator circuit includes a series inductor 10, a series capacitor 12 and a series varactor 14. The anode of the varactor 14 is grounded and the cathode of the varactor 14 receives a bias voltage through an isolation inductor 16. Typically, a bypass capacitor 18 is included in conjunction with isolation inductor 16 to prevent RF energy appearing at the junction between varactor 14 and capacitor 12 (C.sub.12 in the equation below) from being loaded by the V.sub.TUNE circuitry. As the bias voltage is varied, the capacitance of the varactor 14 also varies. The frequency of oscillation is dependent on the reactance of the circuit X.sub.R which may be stated as: ##EQU1## where L is the inductance value of the inductor 10 and C is the total capacitance of the resonant circuit. As the above equations illustrate, varying the capacitance of varactor 14 (C.sub.VARACTOR in the equation above) effectively varies the frequency of resonance of the series circuit.
FIG. 2 illustrates an alternate topology of a resonator circuit known in the prior art. The embodiment of FIG. 2 employs a microstrip line 20 in place of the inductor 10 employed in FIG. 1. This resonator topology provides reasonable noise performance. However, this topology generally suffers from a limited tuning bandwidth of approximately 10%. For a wider tuning bandwidth, the circuit "quality factor" (Q) must be reduced, which in turn degrades noise performance. Therefore, this resonator embodiment requires the selection of either good noise performance or wide tuning bandwidth performance.
FIG. 3 illustrates a resonator circuit known in the prior art which employs open circuit configured coupled microstrip transmission lines. This circuit topology, which is discussed in an article entitled "Noise Calculations and Experimental Results of Varactor Tunable Oscillators with Significantly Reduced Phase Noise", by Guingerich et al., IEEE Transactions on Microwave Theory and Techniques, Vol. 43, No. 2, February 1995, provides broader tuning bandwidth than the single transmission line topology illustrated in FIG. 2. Referring to FIG. 3, this resonator configuration includes a pair of coupled lines 30, 32 separated by a narrow gap. A varactor 14 is electrically connected to one end of a first transmission line 30. The second end of the first transmission line 30 is left in an open circuit configuration, i.e., unconnected. The second transmission line 32 is electromagnetically coupled to the first transmission line 30 as a result of close proximity and parallel arrangement of the lines 30, 32. A first end of the second transmission line 32, which is proximate to the end of the first transmission line 30 connected to the varactor 14, may be connected to a source of bias voltage through a high impedance inductance 34 used to bias the base of a bipolar transistor. The second end of the second transmission line 32 forms the resonator output which is coupled to an oscillator circuit.
This topology suffers from several drawbacks because the transmission lines 30, 32 are connected in an open circuit configuration. This resonator topology suffers from high sensitivity to parasitic loading and radiator loss effects. Further, this resonator structure tends to be inherently capacitive, rather than inductive. In many high frequency oscillator circuits, such as negative resistance oscillators, an inductive resonator is required. Therefore, additional inductive reactance must be added to this resonator topology or the length of the resonator increased. This added length increases the resonator loss and degrades the noise performance of the resulting oscillator.
While various oscillator and resonator topologies are known in the art, there remains a need to provide a resonator circuit and voltage controlled oscillator circuit which provide both wide tuning bandwidth and low noise performance.